1. Field of the Invention
The present invention relates to a read controller for an image pick-up device that controls reading of light signals from an image pick-up device employing an interlace reading system and to an image pick-up apparatus that creates an image signal by capturing object light by the image pick-up device.
2. Description of the Related Art
A charge coupled device (hereinafter, abbreviated as “CCD”) is widely used as an image pick-up device in a digital camera as one of image pick-up apparatuses. The CCD includes a multiplicity of photoelectric transducers disposed in a matrix and color filter arrays are disposed on the multiplicity of photoelectric transducers. Note that the color filter arrays are disposed by a Bayer disposition system, a G-stripe disposition system, an R/G checkered pattern disposition system, a RGB perfect stripe disposition system, and the like. Further, in many cases, the CCD is provided with micro lenses disposed on the color filter arrays in correspondence to the respective photoelectric transducers to increase the amount of light incident on the photoelectric transducers. An R-sensor sensitive to red (R), a G-sensor sensitive to green (G), and a B-sensor sensitive to blue (B) are arranged from the photoelectric transducers, the color filter arrays, and the micro lenses.
There is known a digital camera having a mechanical shutter, a CCD acting as an image pick-up device, and a read controller for the image pick-up device. The CCD reads out (interlace reads) light signals from at least two fields, in which a field for reading a light signal received by an R-sensor sensitive to red light is separated from a field for reading a light signal received by a B-sensor sensitive to blue light, and the read controller controls reading of the light signals from the CCD. In the digital camera, light signals are read from the at least two fields in the state that the mechanical shutter is closed and no light is incident on the camera when exposure is finished. In this case, while charges are being read out by reading a light signal from an initial field (first field), the amount of charges read out by reading a light signal from a next field (second field) is kept by photoelectric transducers. Accordingly, a problem arises in that the charges read out by reading the light signal from the second field leak to an overflow drain, which is disposed to a semiconductor substrate of the CCD to discard excessive charges, in the period during which the charges are being read out by reading the light signal from the first field, and an amount of signal charges accumulated during an exposure period is reduced.
To cope with the above problem, there is proposed a technique for preventing the signal charges, which are read out by reading the light signal from the second field, from leaking to the overflow drain in the period during which the light signal is being read from the first field by setting a potential barrier between a light receiving element and the overflow drain higher in the period during which the first field is read than in the period during which exposure is executed by the light receiving element (refer to Japanese Patent Application Laid-Open No. 11-331708).
Incidentally, there is a phenomenon that the amount of signal charges, which are read out by reading the signal light from the second field, is reduced by the influence of thermal diffusion while the light signal is being read from the first field. Accordingly, it is difficult to keep the amount of signal charges (which is called an amount of saturation charges) read out by reading the light signal from the second field in an maximum possible amount. As a result, the amount of saturation charges reduced by the influence of the thermal diffusion is read, thereby an image signal output from the CCD has a narrow dynamic range, and thus it is difficult to obtain an image of high quality by the digital camera. To cope with the above problem, there is a countermeasure of changing the profile of the CCD, and the like, from which a problem also arises in that a circuit arrangement becomes complex.